Hydrogen fueling stations output hydrogen at high pressures, typically 350 to 700 bar. A significant amount of energy is consumed in operating compressors to reach the output pressure. In some cases, hydrogen is produced at the fueling station by electrolysis of water. Producing compressed hydrogen electro-chemically, in the electrolyser itself, consumes additional energy but less than the energy required for compression to the same pressure. In an energy storage application, producing hydrogen at 60-80 bar may allow hydrogen to be injected into a regional natural gas pipeline without further compression. Accordingly, it is generally desirable to produce hydrogen at above atmospheric pressure. However, increasing cost of the electrolysis unit may make it undesirable to produce hydrogen at the full pressure required for an application. In general, producing hydrogen by electrolysis at a pressure of about 40-80 bar is likely to be cost effective although electrolysis at high pressures, for example 120-350 bar, has also been proposed.
In some cases, both sides of an electrolysis cell are operated under pressure. This creates high-pressure oxygen, which enhances the risk of fire. In other cases, the electrolysis unit is operated at a differential pressure with the oxygen side operating at a lower pressure or even at atmospheric pressure. This is more electrically efficient, avoids producing high-pressure oxygen and allows the oxygen side balance of plant to be made of low-pressure materials.
Differential pressure electrolysis creates mechanical stress across the cell and challenges sealing systems to prevent leaks. Even in non-differential pressure cells, the hydrogen side of the electrolysis cell typically operates at a higher pressure than the oxygen side to avoid leakage of oxygen into the hydrogen. When the overall pressures are large, for example 120 bar or more, preserving this differential while accounting for fluctuations in pressures on either side of the cell can create substantial differential pressures across the cell, at least temporarily.
U.S. Pat. No. 7,432,008 describes an electrochemical cell with a membrane electrode assembly (MEA) located between first and second reactant flow field plates. A seal and gas diffusion layer (GDL) are located between the MEA and each flow field plate. In various embodiments, an edge portion of the GDL is non-porous and overlaps with the seal. The overlap is configured to eliminate or at least minimize a gap that would otherwise exist between the GDL and the seal and thereby counters a problem of having differential pressure distort the MEA into such a gap.